High-pressure discharge lamps as are used, for example, as video projection lamps, generally have two identical electrodes, which are usually in the form of rods. Very disruptive flicker phenomena may arise during operation of such high-pressure discharge lamps on alternating current. These flicker phenomena arise owing to alternating jumping of the root of the arc onto the electrode peaks. This is made possible by the electrode function changing frequently from the anodic phase (positive polarity) to the cathodic phase (negative polarity) at the operating frequency. Such jumping of the arc root in particular impairs
the use of high-pressure discharge lamps in optical devices, for example projection devices, video projectors, microscope lighting and can even result in it not being possible to use these lamps in this application.
U.S. Pat. No. 5,608,294 has disclosed, for low-frequency (50 Hz to a few 100 Hz) operation of a high-pressure discharge lamp, superimposing short synchronous pulses on the square-wave lamp current profile for stabilization purposes, i.e. in order to prevent the root of the arc from jumping. In this case, the current at the end of a half period is increased temporarily prior to subsequent commutation. In accordance with the mentioned document, the current pulse prior to the commutation results in a temporary increase in temperature at the current-conducting roots of the arc on the electrodes, primarily the anode at that time. This results in a material deposition (electrode reformation), i.e. the electrode metal tungsten from the gas cycle process is deposited on the electrodes from the tungsten halides, and a peak is formed on the electrodes, which stabilizes the discharge and the root of the arc very effectively.
WO 03/098979 A1 has disclosed the operation of a high-pressure discharge lamp with an unmodulated RF signal of more than 3 MHz. In general, high-pressure discharge lamps permit successful RF operation only above frequencies which are above the acoustic resonances in the combustion chamber. These acoustic resonances result in strong flows in the combustion chamber which generally considerably disrupt the discharge arc. However, the literature contains attempts to damp
the acoustic resonances by means of suitable feed currents or to completely avoid said resonances. By way of example, reference is made to DE 10 2005 028 417.5 and DE 10 2005 059 763.7. Such solutions are usually very complex, however.
Finally, reference is made to DE 198 29 600 A1, which is concerned with RE operation of a high-pressure discharge lamp. It relates in particular likewise to the problem of the jumping of the root of the arc onto the electrode peaks. Against the background of a prior art in which the high-pressure discharge lamps were operated at a frequency of below 2 kHz, said document proposes the solution of operating the lamp at a frequency above 800 kHz, preferably above 1 MHz and particularly preferably between 2 and 3 MHz. In a preferred development, the operating frequency is wobbled both continuously and suddenly with a modulation frequency of less than 10 kHz, preferably between 1 and 2 kHz. Although this can under some circumstances provide a solution for certain high-pressure discharge lamps, this measure has proven to be ineffective in the case of the high-pressure discharge lamps investigated by the inventors of the present invention.
The basic solution of preventing jumping of the root of the arc onto the electrode peaks during RF operation of a high-pressure discharge lamp is provided in the subsequently published patent application PCT/EP2006/068269 by the same applicant as the present application. The solution consists in the electronic ballast further being designed to modulate the AC feed signal in terms of its amplitude.
The present application is aimed at a preferred use sector of such high-pressure discharge lamps: the known term DLP (digital light processing) is used to describe a technology which is used in video projectors and rear-projection televisions. It is based on microscopically small mirrors which are fitted on a DMD (digital micromirror device) chip. In this case, the mirrors are smaller than a fifth of the width of a human hair. They have two stable end states, between which they can alternate within 16 μs in a preferred embodiment. The movement is brought about by the force effect of electrostatic fields. Owing to the incline of the individual micromirrors on the DMD chip, the light is either reflected directly towards the optical unit or directed towards an absorber. By pulse-width-modulated driving of the mirrors, various brightness levels of the individual pixels can be generated.
DMD chips with an XGA image resolution of 1024×768 contain an arrangement of 786,432 tiny mirrors. In the meantime, DMD chips with resolutions of up to 2048×1080 can be obtained, i.e. approximately two million mirrors.
Since the DMD chips reflect the white light of a projection lamp, additional steps are required for a colored image. In a 1-chip projector, a color wheel is connected into the optical path in front of the DMD chip, with color filters of the primary colors (generally the colors red, green and blue, but sometimes also other colors as well) rotating on said color wheel. In order to achieve improved brightness values in the white region, white is also added to the color wheel. With the position of the color filter, the electronics change
the partial image which is reflected by the DMD. Owing to the rotational speed of the color wheel and the inertia of the human eye, the partial images are added to form a colored image impression. Since the detection frequency is different from human to human, there were reports, primarily in the case of the first models, of a so-called rainbow effect, which occurred when the viewer perceived the individual colors. In a further step, the revolution number of the wheel was therefore doubled and the number of color segments increased in the case of more recent models.
The basic design of such a projection apparatus is provided, for example, in U.S. Pat. No. 5,917,558. FIG. 2 of said document U.S. Pat. No. 5,917,558 shows various pulse control modes for the projection lamp. As can be seen from said figure, these pulses are LF pulses, with a modulation period comprising a serial sequence of a plurality of signal sections which are associated with different colors, in a time range. If a high-pressure discharge lamp is used as a projection lamp, unfortunately the abovementioned undesirable effect of the jumping of the root of the arc onto the electrode peaks occurs during operation with such pulse trains.
Moreover, with such operation, a noticeable dip in the luminous flux and therefore a loss of control of the luminous flux in this period arise despite the rapid commutation of the lamp. This loss of control needs to be avoided in present-day applications by this period being placed in blanking intervals. Furthermore, the procedure in accordance with the prior art displays oscillation phenomena of the luminous flux after the dip in the luminous flux. In this period, the luminous flux can therefore not be controlled and often also cannot be used. The loss of control in modern-day applications disrupts, for example, the color balance and needs to be compensated for by complicated measures in the device.
Further prior art can be found in U.S. Pat. No. 5,109,181, DE 100 18 860 A1 and US 2006/0022613 A1.